Double crucible crystal growing apparatus

ABSTRACT

The disclosure is directed to a double crucible Czochralski crystal semiconductor growing apparatus (10). An inner crucible (14) floats in a melt within an outer crucible (13) and a single crystal semiconductor billet (23) is pulled from the melt (16) in the inner crucible. An elongated tubular member (26), having at least one small aperture (33) in the wall thereof, provides a channel between the outer and inner crucibles. The tubular member (26) permits flow of the melt in the outer crucible (14) to the inner crucible (13) while inhibiting the diffusion of dopant material from the inner to outer crucible while any gas in the member will pass through the aperture (33).

TECHNICAL FIELD

The invention is directed to an apparatus for producing semiconductorsingle crystal material using the Czochralski process. In particular,axial dopant uniformity is achieved in the semiconductor material usingfirst and second crucibles having an interconnecting channel to permitflow of semiconductor melt therebetween.

BACKGROUND OF THE INVENTION

It is well known to produce semiconductor single crystal material usingthe Czochralski technique by forming a melt of the crystal material andbringing a "seed crystal" into contact with the melt. The seed is thenpulled slowly upwards, the molten material solidifying at the seed-meltinterface, thus forming a single crystal billet.

Such a method has been found to be most effective, however, the crystalproduced suffers from non-uniform electrical resistivity along itslength. This is primarily due to the fact that the doping agents (e.g.,arsenic, antimony, gallium or indium) commonly added to the puresemiconductor material (e.g., silicon, germanium), are more soluble inthe liquid semiconductor material. Hence, in a growing crystal, theconcentration of doping agent in the solid semiconductor crystal is lessthan the concentration of doping agent in the adjacent liquid melt.Therefore, as a crystal billet is grown by the Czochralski process, asteadily increasing concentration of doping agent is left in theremaining melt resulting in an increase in resistivity along the lengthof the grown crystal billet.

Although such a crystal billet may be used in many applications wherechanges in resistivity are not critical, a number of devices, such astransistors, have parameters which vary more or less linearly with theresistivity of the semiconductor single crystal. The difficulty ofmaking devices with predictable characteristics is greatly increased ifuniform resistivity is not obtained during the crystal growing process.

A technique used to improve the axial resistivity uniformity of singlecrystal semiconductor billet is described in U.S. Pat. No. 2,944,875which issued on July 12, 1960. That patent describes an apparatus inwhich the concentration of doping agent as well as the volume of thecrystal melt is kept substantially constant throughout the seed-pullingprocess so that a single crystal billet of uniform resistivity may begrown. The apparatus comprises a pair of cylindrical crucibles thesecond of which is designed to fit loosely into the first and the secondor inner crucible is provided with a small hole drilled through thebottom thereof. The crucibles are so arranged that a first charge ofhigh purity undoped or lightly doped semiconductor material is placed inthe outer crucible and a second charge of highly doped semiconductormaterial placed in the inner crucible. Heat is then applied to thedouble crucible arrangement to form a crystal melt in the inner andouter crucibles. Accordingly, the outer crucible contains a first meltof lightly doped or undoped semiconductor material and the innercrucible, floating within the first melt, holds a second melt ofsemiconductor material having a substantially greater concentration ofdopant therein.

A single crystal billet may be grown from the inner crucible melt by theabove-described seed-pulling method and, as the crystal grows, thelightly doped semiconductor material will flow from the outer crucibleinto the inner crucible, through the hole to maintain the floating innercrucible at an equilibrium level while diluting the dopant concentrationof the melt therein. Therefore, until the inner crucible touches thebottom of the outer crucible, the volume of liquid semiconductormaterial in the inner crucible will remain constant. Since only a smallfraction of the dopant agent is used up in the growing crystal billet,the dopant concentration level in the inner crucible melt remainssubstantially constant during the process which tends to result in thecrystal having uniform resistivity along its length.

However, the single opening or hole in the inner floating crucibleundesirably permits mechanical mixing of the melt in the two cruciblesas well as back dopant diffusion (i.e., diffusion of dopant from theinner to the outer crucible) which alters the dopant concentrations inthe inner and outer crucibles and results in non-uniform resistivity ofthe grown single crystal billet. In order to decrease any back diffusionor mechanical mixing of the inner crucible melt and the outer cruciblemelt, a narrow channel may be formed between the inner and outercrucibles as shown in an article titled "A Process for Obtaining SingleCrystals with Uniform Solute Concentrations," by A. V. Valcic, in SolidState Electronics, 1960, pages 131 to 134.

However, as the solid semiconductor material is simultaneously melted inthe crucibles, gas bubbles tend to be captured in such a narrowpassageway. The captured bubbles stop the melt flow between thecrucibles causing the concentration of the dopant in the inner crucibleto rapidly increase requiring that the process be aborted or that thegrown billet be used for fabrication of devices having less stringentresistivity standards.

SUMMARY OF THE INVENTION

The instant apparatus overcomes the foregoing problem with an apparatuscomprising first and second chambers which have at least one hollowinterconnecting member communicating therebetween as melt is withdrawnfrom one of the chambers. The interconnecting member has a smallaperture in the wall thereof to permit the escape of any gases lodgedwithin the member while substantially precluding the flow of melttherethrough.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a partial cross-sectional view of a prior art crystal growingapparatus;

FIG. 2 is a partial cross-sectional view of a floating crucibleCzochralski crystal growing apparatus in accordance with the instantinvention;

FIGS. 3A, 3B and 3C depict the melting of semiconductor nugget materialin a floating crucible apparatus in schematic form;

FIGS. 4 and 5 are cross-sectional views of the instant interconnectingmeans; and

FIGS. 6A, 6B and 6C depict, in schematic form, alternative doublecrucible apparatus in which the instant invention can be advantageouslyimplemented.

DETAILED DESCRIPTION

The instant invention is described in a particular exemplary embodimentin which a floating double crucible arrangement is used to grow singlecrystal silicon billet. However, such an embodiment is for purposes ofexposition and not for limitation. Various other embodiments arecontemplated wherein first and second chambers containing melts arearranged so that the melt from one chamber must flow into the otherchamber via a channel communicating therebetween.

FIG. 1 depicts a prior art floating double crucible Czochralski (Cz)semiconductor crystal growing apparatus, generally indicated by thenumeral 10. The crystal growing apparatus 10 is comprised of housing 11(e.g., graphite) surrounded by a series of high frequency inductioncoils 12--12. An outer crucible 13 is seated within the housing 11 andan inner crucible 14 is shown floating in a melt (e.g., germaniumsilicon). The first melt 16 within the inner crucible 14 has a firstdopant concentration level and the second melt 17 has a second dopantconcentration level which is lower than the first concentration level.An opening 18 in the bottom of the inner crucible 14 permits flow of thesecond melt 17 into the inner crucible 14.

In operation, a rotating pull rod 21 holding a single crystalsemiconductor seed 22 causes the seed to contact the surface of thefirst melt 16 and the pull rod is moved upward to draw a single crystalsemiconductor billet 23 from the melt. As the single crystalsemiconductor billet 23 is being drawn from the first melt 16 portionsof the second melt 17 pass through the opening 18 into the innercrucible 14 to (1) replenish the melt used to form the crystal billet 23and (2) maintain the dopant concentration in the first meltsubstantially constant by diluting the higher dopant concentration innercrucible melt 16 with the lower concentration melt 17 in the outercrucible.

Although the above process operates effectively, it has been found thatthere is some mechanical mixing and back diffusion from the first melt16 in the inner crucible 14 into the second melt 17 in the outercrucible 13 via the opening 18. Such mixing and back diffusion altersthe dopant concentrations of the melts 16 and 17 which ultimatelyadversely affects the resistivity of the grown single crystalsemiconductor billet 23 resulting in a non-uniform axial resistivity ofthe grown billet 23.

FIG. 2 depicts an exemplary crystal growing apparatus 10 which has beenmodified to incorporate the instant inventive concepts which overcomethe foregoing problem. The inner crucible 14, in particular, has beenmodified to insert a narrow elongated member 26 through the wall of theinner crucible to permit communication between the first and secondmelts 16 and 17. In the exemplary embodiment, the member 26 is a hollowquartz tube, however, other cross-sectional geometries may be used(e.g., rectangular, triangular, etc.).

As hereinbefore indicated, the first and second melts 16 and 17 aredoped at different concentration levels and a concentration gradient,dc/dl, exists across the length of the tubular member 26. This gradientis inversely proportional to the length of the member 26 and directlyproportional to the doping level when the concentration ratio of the twocontainers is fixed. At the static, or no flow, condition, dopantdiffusion may be approximated as:

    Jα(D (dc/dl))·A                             (1)

where:

J=dopant atom transfer rate;

A_(t) =cross-sectional area of the tube 26; and

D=diffusion coefficient of dopant in the melt. Accordingly, thecontrolling factors are the dimensions of the member 26 and the dopinglevel. A long and narrow interconnecting member 26 is preferred fordiffusion suppression. A low doping level and, therefore, a lowconcentration gradient dc/dl results in less diffusion than in the caseof high doping levels.

The average time, t, for a dopant atom to jump across the member 26 oflength, l, is related to the diffusion coefficient, D, by the followingequation: t=l² /D. When the billet 23 is pulled from the inner crucible14, the pressure difference between the inner crucible and the outercrucible 13 will drive the second melt 17 into the first melt 16 with afinite flow rate, V_(t), which is opposite to the diffusion direction.The flow, therefore, aids in suppressing the back diffusion of thedopant from the first melt 16 to the second melt 17. To sufficientlysuppress the dopant transfer, the criterion is:

    V.sub.t l>>D                                               (2)

The flow rate, V_(t), is related to the crystal pull rate V₁, the crosssectional area, A_(s), of billet 23; and the cross section, A_(t), ofthe tube 26 as follows:

    V.sub.t =V.sub.1 (A.sub.s /A.sub.t).                       (3)

Then the inequality (2) may be written as:

    lV.sub.1 (A.sub.s /A.sub.t)>>D                             (4)

The value of D is on the order of 10⁻⁴ cm² /sec for commonly useddopants in silicon.

The more important diffusion consideration occurs when there is no flowthrough the member 26. This corresponds to the necking and shoulderingpart of the crystal growth process where no significant material iswithdrawn from the inner crucible 14. A similar situation exists whencrystal growth is aborted due to losing dislocation free structure, andbefore another billet 23 is pulled. Here diffusion control depends on asufficiently long and narrow connecting tube 26 to minimize dopanttransfer for the duration of the no-flow part of the growing process.

The most convenient procedure for forming the first and second melts 16and 17 is to melt down semiconductor nuggets 31--31 as shownschematically in FIGS. 3A to 3C. The nuggets 31--31 are placed in boththe inner crucible 14 and the outer crucible 13 as shown in FIG. 3A. Theheating coils 12--12 (see FIG. 2) are energized and the nuggets 31--31start to melt (FIG. 3B) until a melt has been formed in both crucibles13 and 14 (FIG. 3C). As indicated in FIG. 3C, often, a gas bubble 32 istrapped within the tube 26 which will prevent the flow of melt betweenthe crucibles 13 and 14 requiring that the run be aborted.

FIG. 4 is an enlarged cross section (not to scale) of the tube 26 havinga gas bubble 32 formed therein. In order to eliminate the problem oftrapping bubble 32, a narrow aperture 33 is placed through the wall ofthe member 26. As can be seen in FIG. 5, the trapped gas forming thebubble 32 will pass through the narrow aperture 33 and float to the topof the melt where it will burst. The cross-sectional area of theaperture 33 is small enough so that the surface tension of the melt willhot wet or penetrate the aperture but will permit the gas to passtherethrough. The aperture 33 may have any desired cross-sectionalgeometry (e.g., circular, triangular, rectangular, etc.) with the areabeing such that substantially no molten material will pass therethrough.The cross-sectional area of the aperture 33 will depend on the surfacetension of the melt as well as the potential pressure differential thatmay be generated between the melts 16 and 17. In instances where thesurface tension of the molten material is very low, some of the materialmay pass through the aperture 33. However, the amount of melt passingthrough such a small aperture has substantially no affect on theresistivity of the grown billet 23.

In a particular working embodiment in which the melt was silicon havinga surface tension of approximately 720 dynes/cm². The member 26 was ahollow tube made of quartz and the aperture 33 in the wall of the memberwas substantially circular in cross section with a diameter ofapproximately 0.3 mm.

Preferably, the aperture 33 should be located through the top wall ofthe member 26 to facilitate the escape of the gas therethrough, however,if necessary, the aperture may be placed at other positions, includingthe bottom of the member. In addition, the number of connecting members26 as well as the number of apertures 33 therein is not limited. Anumber of members 26 each having a plurality of openings have beeneffectively used to permit trapped gas to flow therethrough whilesubstantially confining the melt therein.

Although the instant invention has been described in relation to its usein a floating double crucible arrangement (FIG. 2), it is not solimited. The invention has application wherever there is a semiconductormaterial flowing through a communicating member between chambers and gasbubbles can be trapped in that member. FIGS. 6A to 6C are illustrativeof, but not limited to, three different arrangements where the instantapertured connecting member 26 may be effectively used. FIG. 6A depictsa double crucible arrangement wherein the inner chamber 14 is fixedlymounted within the outer chamber 13. FIG. 6B is arranged to have theinner chamber fixedly mounted on brackets 52--52 as the outer chamber 13is moved upward. Additionally, FIG. 6C depicts an outer chamber 13having a dividing wall 51, each example makes use of the instantapertured interconnecting member 26.

It should be clear that the instant invention should not be limited toan apparatus comprising two chambers. An arrangement of a multiplicityof chambers, each adapted to contain a semiconductor melt andinterconnected by a plurality of communicating members, is contemplatedto fall within the instant concepts.

What is claimed is:
 1. An apparatus for growing a single crystalsemiconductor billet, comprising:first and second chambers, each adaptedto contain a semiconductor melt; and a hollow interconnecting member,communicating between the chambers to permit flow of the melt from thefirst to the second chamber and being of such a length as to inhibitback diffusion of the melt therethrough, the member having at least oneaperture extending through the wall thereof to permit the escape of gaswithin the member while substantially precluding the flow of melttherethrough.
 2. An apparatus for growing a single crystal semiconductorbillet, comprising:a plurality of crucibles, each adapted to contain amelt of the semiconductor material; and hollow interconnecting members,communicating between the crucibles to permit the melt to flow betweenthe crucibles as the billet is grown from the melt in one of saidcrucibles, each member being of such a length as to inhibit backdiffusion of the melt from the growing crucible, the members each havingat least one aperture in the wall thereof to permit the escape of anygases within the members while substantially precluding the flow of themelt therethrough.
 3. The apparatus as set forth in claim 1 or 2wherein:the semiconductor melt is silicon; and each hollowinterconnecting member is a quartz tube with a circular aperture in thewall thereof having a diameter of 0.3 mm.
 4. An apparatus for growing asingle crystal semiconductor billet comprising:first and secondcrucibles sharing a common wall and each adapted to contain dopedsemiconductor melt; an elongated, hollow quartz tube passing through thecommon wall and being of such a length so as to permit passage of thesemiconductor melt from the first to the second crucible whileinhibiting back diffusion of dopant material therebetween; and the tubehaving at least one aperture in the wall thereof to permit gas to passwhile substantially precluding the flow of the melt therethrough.
 5. Theapparatus as set forth in claim 4, wherein:the semiconductor melt issilicon; and the aperture is substantially circular with a diameter of0.3 mm.